The present invention relates to the detection of line-over defects at the finish of glassware containers, and more particularly to the use of optical apparatus for this purpose.
The so-called "line-over" defect is a common problem in the manufacture of glass containers; other common names for line-overs are "shear marks" and "nit marks". This defect is a very small groove which extends radially across the sealing surface of the finish of the glass container; see FIG. 1. It has been considered one of the worst and hardest to detect defects throughout the industry. In carbonated beverages, for example, the cap seal is not capable of sealing these fine grooves, and over a period of time, the carbonation gases seep out and the beverage goes flat. Food jars are another critical area. If the seal is bad due to this defect, the food can become contaminated. The baby food industry is particularly concerned with line-overs.
It is believed that this groove is caused by a line of cold glass left on the molten gob of glass where the previous gob was sheared as the glass streams from the feeder through the shears. See, for example, U.S. Pat. No. 4,515,002.
In recent years, many attempts have been made to develop equipment to sense line-over defects in glass containers in order to reject bad bottles from the production line with a high degree of accuracy. Most such devices cause an undesirably large number of good ware to be rejected while still missing a considerable percentage of defective containers. Prior art approaches have included mechanical sensing arrangements such as those disclosed in U.S. Pat. Nos. 3,879,993 and 3,395,573. A wide variety of optical approaches have also been taken. One approach uses specular, focused light directed at the finish of the container from within, wherein the presence of line-over defects will cause the focused light to be refracted in a direction different from that in the absence of a defect, this deflection being detected by strategically placed photosensors. See, for example, U.S. Pat. Nos. 3,302,787 and 3,107,011. Another approach, disclosed in U.S. Pat. No. 4,606,634, uses a planar source of diffuse light, a video camera being used to produce an image of any line-over defects.
A third approach, generally of the type adopted in the present invention, uses light downwardly directed at the sealing surface, and detects reflected light in order to sense whether or not a line-over defect is present. The system of U.S. Pat. No. 3,880,750 focuses an intense spot of incandescent light across the container rim, and uses an optical sensor assembly to receive reflected light, together with processing electronics to detect departures from a normal light signal. Frequency filtering of the photodetector signal removes the normal light signal and provides individual signals for different defect types. The light source and photodetector assemblies are located at complementary acute angles relative to the plane of the sealing surface.
Another patent which discloses a system utilizing the reflected-light approach, commonly assigned with the present application, is U.S. Pat. No. 4,488,648. This patent preferably utilizes a DC light source at an acute angle relative to the sealing surface, the photodetector assembly also being located at an acute angle. (See FIG. 8.) A line-over defect produces a reduction in light - a "dark spot". Both this system and the '750 discussed above did not successfully solve certain problems posed by the use of reflected light for line-over defect detection. The sidewall angle of line-over grooves is completely unpredictable. One wall's angle may be steep while the opposing wall is shallow. It is difficult to reliably reflect light back onto a sensor from these defects as they vary from one container to the next and are very unpredictable. FIGS. 2A, 2B, and 2C illustrate three typical line-over groove orientations, as seen from the bottle interior. Further problems with this reflected light approach include dirt, dust, and rough finish surfaces that can cause light to be reflected back to the sensor as well.
Accordingly, it is a principal object of the invention to provide a more accurate line-over detection system of the optical type. A related object is to improve the percentage of line-over defects which are detected. Another related object is to reduce the percentage of "false positives", for example, due to dust, dirt, and rough finish surfaces.